Method for operating a logic module refrigeration unit

ABSTRACT

A refrigeration system for cooling a logic module includes an evaporator housing including an evaporator block in thermal communication with the logic module. The evaporator housing includes a humidity sensor for detecting a humidity within the evaporator housing. The system further comprises a controller for controlling a refrigeration unit supplying cold refrigerant to the evaporator block in response to the operating conditions of the logic module and the temperature of the evaporator block. In another aspect of the invention, two modular refrigeration units are independently operable to cool the evaporator block, and each refrigeration unit is controllable in various modes of operation including an enabled mode in which it is ready to cool the evaporator and an on mode in which it is actively cooling the evaporator. In another aspect of the invention, the evaporator block and a heater on a reverse side of the circuit board are particularly controlled during concurrent repair operations. In another aspect of the invention, faulty sensors are recognized as such and an appropriate response is made. In another aspect of the invention, the system is shut down in a manner allowing rapid access to the logic module.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional of U.S. patent application Ser.No. 09/896,610 filed Jun. 29, 2001, the contents of which areincorporated by reference herein in their entirety.

BACKGROUND

[0002] The present invention relates to software for a cooling andcondensation control system. In particular, the present inventionrelates to a cooling system and condensation control system for computerlogic modules.

[0003] One of the factors that limit processing speed in computersystems is the generation of excessive heat at higher clock speeds.Significant gains of speed and reliability have been achieved by coolingcomputer logic modules down to temperatures below ambient.Unfortunately, cooling a logic module to below ambient temperatures canresult in the formation of condensation, which is undesirable in acomputer system.

[0004] Prior attempts at providing a cooling system for a computermodule have not been satisfactory for higher-end computing applications.For example, one approach has been to remove moisture from incoming aircooled to 5° C. This approach requires handling a tremendous amount ofwater, and does not prevent condensation in an application whererefrigerant may be operating as cold as −40° Celsius. Another approachhas been to simply apply a fixed high-power heater around an evaporatorunit which surrounds the logic module. In this way, the surfacetemperature of the logic module housing remains above the dew point.Another approach relies on enclosing the logic module in a vacuumenclosure as a means of providing effective insulation. Unfortunately,these approaches cannot adequately ensure that there will be nocondensation in the evaporator housing and are therefore notsufficiently reliable.

[0005] Another problem unresolved by prior art cooling systems relatesto condensation formed on the opposite side of the circuit board. Thisproblem has limited the temperatures to which the logic module can becooled to avoid condensation.

SUMMARY

[0006] The above discussed and other drawbacks and deficiencies of theprior art are overcome or alleviated by a refrigeration system includingan evaporator housing including an evaporator block in thermalcommunication with the logic module. The evaporator housing includes ahumidity sensor for detecting a humidity within the evaporator housing.The system further comprises a controller for controlling arefrigeration unit supplying cold refrigerant to the evaporator block inresponse to the operating conditions of the logic module and thetemperature of the evaporator block. In another aspect of the invention,two modular refrigeration units are independently operable to cool theevaporator block, and each refrigeration unit is controllable in variousmodes of operation including an enabled mode in which it is ready tocool the evaporator and an on mode in which it is actively cooling theevaporator. In another aspect of the invention, the evaporator block anda heater on a reverse side of the circuit board are particularlycontrolled during concurrent repair operations. In another aspect of theinvention, faulty sensors are recognized as such and an appropriateresponse is made. In another aspect of the invention, the system is shutdown in a manner allowing rapid access to the logic module.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The above-discussed and other features and advantages will bemade known from the following detailed description and accompanyingdrawings, wherein like elements are numbered alike, and in which:

[0008]FIG. 1 is a schematic representation of an exemplary embodiment ofthe hardware components of a refrigeration system;

[0009]FIG. 2 is a schematic representation of an exemplary refrigerationunit and evaporator block;

[0010]FIG. 3 is a cross-section view of an exemplary installedevaporator and heater;

[0011]FIG. 4 is a front perspective view of the evaporator of FIG. 3;

[0012]FIG. 5 is a rear perspective view of the evaporator of FIG. 3;

[0013]FIG. 6 is a schematic diagram depicting air flow management schemein a processor cage;

[0014]FIG. 7 is a perspective view of an exemplary processor cage;

[0015]FIG. 8 schematically represents certain components of arefrigeration unit controller; and

[0016]FIGS. 9, 10, and 11 show flow charts representing certainexemplary methods for operating or controlling a refrigeration system.

DETAILED DESCRIPTION

[0017]FIG. 1 is a schematic diagram providing an overview of thehardware components of exemplary refrigeration system 10. Refrigerationsystem 10 includes redundant refrigeration units 20 (two are shown) thatsequentially or in unison provide coolant to evaporator 30 viarefrigerant lines 26. Although the cooling system will be described withreference to two refrigeration units 20, it will be understood that theinvention can be adapted for use with one or more such refrigerationunits. Evaporator 30 is disposed over logic module 50 and is in thermalcommunication therewith for cooling logic module 50 to temperaturesbelow ambient. Heat from logic module 50 is absorbed by coolant inevaporator 30. Coolant is then passed back to refrigeration units 20where it is compressed, condensed, and expanded in any knownrefrigeration cycle.

[0018] Logic module 50 is preferably a multi-chip module, as aregenerally known in the art, but cooling system 10 is applicable to otherconcentrated heat generating devices that are advantageously maintainedat below ambient temperatures. Logic module 50 is electrically andmechanically attached to a first side 58 of circuit board 55. Both logicmodule 50 and circuit board 55 are relatively flat structures havingheights that are substantially less than their widths or lengths, andare shown in FIG. 1 in profile view (i.e., disposed perpendicular to thepage). To prevent condensation on a second side 59 of circuit board 55from forming, a heater 40 is disposed on second side 59 of circuit board55. Heater 40 includes a heat spreader plate 41 in thermal communicationwith circuit board 55. Thermal pads 52 (FIG. 3) enhance thermal couplingbetween heat spreader plate 41 and plated through-hole connections (notshown) of circuit board 55, effectively warming circuit board 55. Heater40 further includes redundant heat cartridges 44 to provide heat to heatspreader plate 41. Each heat cartridge 44 includes a resistive heatelement (not shown) for converting electrical energy into heat energy.In a preferred embodiment, each heat cartridge is rated at 300W. Inaddition to heat cartridges, other heat source types, such as flatresistive heaters, are contemplated.

[0019] Each refrigeration unit 20 includes a respective controller 24 incommunication with a primary cage controller 60 and secondary cagecontroller 65, the latter being available in case of failure of primarycage controller 60, via two-way communication lines 64. Controller 24operates in response to instructions from primary cage controller 60 orsecondary cage controller 65 to cool logic module 50 to a desiredtemperature. In aid of this objective, controller 24 is equipped withvarious outputs and inputs to control refrigeration equipment 22 and tomonitor the actual temperature of the evaporator. In addition to coolinglogic module 50, controller 24 is responsible for preventingcondensation within evaporator 30 and on circuit board 55. monitoringthe temperature and relative humidity in the evaporator cavity (to bedescribed in more detail below) and the temperature of heater 40, toensure that it is greater than the dew point within the processor cage(FIGS. 6, 7).

[0020] Controller 24 controls refrigeration equipment 22 withinrefrigeration unit 20 via signal lines 23, and controls heat cartridge44 via lines 29. Evaporator plate temperature is provided via lines 36and evaporator cavity temperature and relative humidity are provided vialines 33 and 34, respectively. Finally, lines 42 provide heatertemperature feedback.

[0021] Refrigeration units 20 are modular in nature, i.e., they areinterchangeable and provide redundancy or backup capabilities toevaporator 30. Refrigerant lines 26 and refrigerant return lines 27 areconnected to refrigeration unit 20 via respective quick-disconnectcouplings 25 so that either refrigeration unit 20 may be quickly swappedout for maintenance or replacement once another refrigeration unit is online. Refrigeration units 20 each include a condenser and compressor, asis known in the refrigeration art, and passes pressurized refrigerant,which may exist as a liquid, gas, or mixed-phase fluid to evaporator 30via refrigerant lines 26. In a preferred embodiment, the compressor is aManeurop LT-22 compressor, but any rotary, reciprocating, or scrollcompressor of sufficient cooling capacity could be used. The preciserefrigeration cycle and working fluid is determined based on a number offactors, such as the desired resultant temperature,environmental/regulatory considerations, desired coefficient ofperformance, desired size of condenser, cost, etc.

[0022] Referring now to FIG. 2, an exemplary cooling cycle using R507(AZ50) refrigerant provides temperatures as cold as −40° C. and colder.As would be appreciated by those skilled in the art, the targettemperature may vary with the compression ratio and other factors,including various control measures as will be hereinafter described.

[0023] In the exemplary arrangement shown, a mixture of gas and liquidrefrigerant are passed to evaporator block 31 via refrigerant line 26.Evaporator block 31 comprises copper, aluminum, or other heat conductivematerial and includes independent serpentine paths 61 and 62 formedtherein. Serpentine paths 61 and 62 are circuitous pathways that may beformed by any known method, such as by cutting paths into a center layerthat is then sealed between upper and lower layers of the material.Exemplary evaporator block structures are shown and described incommonly assigned U.S. Pat. No. 5,970,731 to Hare et al, issued Oct. 26,1999 and U.S. Pat. No. 6,035,655 also to Hare et al., issued Mar. 14,2000, both of which are herein incorporated by reference. Evaporatorblock 31 includes an additional serpentine path 62 for connection with asecond refrigeration unit 20. Serpentine paths 61 and 62 are separatedand are not in fluid communication with each other so that onerefrigeration unit 20 may be disconnected while the other continues tooperate. Evaporator block 31 is disposed in thermal communication withlogic module 50 within evaporator 30 (FIGS. 1, 3) so that heat fromlogic module 50 is readily absorbed by refrigerant in serpentine paths61 and 62.

[0024] Upon absorbing heat from logic module 50 (FIG. 1) the refrigerantvaporizes and is passed back to refrigeration unit 20 via refrigerantreturn line 27. This gaseous refrigerant is passed to compressor 70 vialine 63, compressed therein, and then passed to condenser 71 via line67, wherein it is condensed back into a liquid or mixed phase fluid.Blower 72 is driven by a variable A.C. motor or D.C. motor and pulls airthrough electronic controller 24 and condenser 71 in a path generallyrepresented by arrow 88 for absorbing the heat from the refrigerant.

[0025] After being condensed in condenser 71, the coolant is received infilter/drier tank 78 via line 76, then passed to expansion valve 85where a portion of the coolant is vaporized and its temperature isreduced. The coolant is then once again made available to evaporator 30via line 87. A bypass line 79 is provided between lines 67 and 87 forbypassing the condenser and expansion valve in order to moderate thecoolant's temperature. Bypass line 79 includes hot gas bypass valve 80,actuated by solenoid 82.

[0026] Controller 24 includes a processor and memory, as will be furtherdescribed, for executing machine code to control the operation ofrefrigeration unit 20. Control unit 24 includes various inputs fortemperature and humidity sensors in evaporator 30, to be described inmore detail below, and outputs to control compressor 70, bypass valve80, and blower 72 via lines 66, 83, and 73, respectively. Any of lines66, 83, and 73 may carry control signals for actuating a relay or motorcontroller (not shown) which switches on and off power to the respectivecomponent, or such relay/motor controller may be internal to controller24, and lines 66, 73, and 83 carry power to the respective components,in the known manner. The power supply and power lines are notillustrated for sake of clarity.

[0027] Additionally, refrigeration unit 20 includes outputs forcontrolling heat cartridge 44 (FIGS. 1, 3) and an input for receivingelectrical power for supplying energy to controller 24, as well ascompressor 51, blower 61, and solenoid 82.

[0028]FIG. 3 shows a cross section view of an exemplary implementationof an evaporator 30 and heater 40 assembled with a logic module 50 andcircuit board 55. Circuit board 55 is provided with stiffeners 97 oneither side to aid in supporting and attaching evaporator 30 and heater40 to circuit board 55. Evaporator 30 is attached to a first side 58 ofcircuit board 55 and heater 40 is attached to a second side of circuitboard 55 by screws 43 connected to stiffener 97 as shown. Refrigerantline 26 and refrigerant return line 27 supplies one of two serpentinepaths 61 and 62 (FIG. 2). A second pair of refrigerant and refrigerantreturn lines lie directly behind lines 26 and 27 shown, allowingconnection to an additional refrigeration unit 20, as described above.

[0029] Refrigerant line 26 and refrigerant return line 27 are connectedto evaporator block 31 which is in thermal communication with logicmodule 50. Evaporator block and logic module 50 are sealed in anevaporator housing 32 by gasket 94. After assembly, Evaporator cavity 35is filled with insulation, such as an injected polymer foam insulation,to reduce the amount of heat absorbed from evaporator housing 32, thuspreventing the temperature of evaporator housing 32 from dropping belowdew point. Grommet 48 is formed of insulating material and includesholes for passing refrigerant lines 26 and refrigerant return lines 27into evaporator housing 32. Also extending into housing 32 is evaporatorblock temperature probe 39, which includes a sensor extending intoevaporator block 31 for detecting the temperature of the evaporatorblock, and providing feedback information to refrigeration units 20 aspreviously described. Evaporator block temperature probe 39 preferablycomprises dual thermistors for providing independent temperaturedetection for each of two refrigeration units 20. Evaporator housing 32is attached to circuit board 55 in the known manner, with evaporatorblock 31 being biased against logic module 50 using biasing elements 49,which may comprise metal springs or elastomeric blocks.

[0030] Heater 40 comprises heat spreader plate 41 which includesaccommodations for two heat cartridges 44 and dual temperature sensors46 for detecting the temperature of heat spreader plate 41 and providingfeedback to respective refrigeration units 20.

[0031] The internal atmosphere of evaporator 30 will be described withreference now to FIGS. 1 and 3. Desiccant canister 45 is connected atits inlet to a source of pressurized air. Dry air flows out of desiccantcanister 45 through capillary tube 47 to heat spreader plate 41. Heatspreader plate 41 and circuit board 55 include an aligned through-hole95 for conducting dry air from said capillary tube to the first side 58of circuit board 55. Logic module 50 comprises a zero-insertion forceconnector as is known in the art which includes a small air space 57(FIG. 3) between logic module 50 and circuit board 55. This arrangementraises the air pressure in evaporator housing 32 to slightly aboveambient. Thus, any small amount of leakage or diffusion will only resultin dry air infiltrating into evaporator housing 32.

[0032] Evaporator 30 also includes a desiccant slot 37 housing adesiccant bag 92 which absorbs any remaining moisture in evaporatorcavity 35, such as might occur upon replacement or servicing of dualhumidity and temperature sensor 38. Arrows 96 shows free movement of airbetween evaporator cavity 35 and desiccant slot 37. Desiccant slot 37also houses dual humidity and temperature sensor 38, which includesredundant humidity and temperatures sensors. In a preferred embodiment,humidity and temperature sensor is or is similar to one available fromHoneywell, part number HIH-3602-C and includes two independent humiditysensors and two independent thermistors. Capillary tube 47 issufficiently long or otherwise includes an airflow resistor, such as anorifice, to prevent excessive air flow through capillary tube 47 in thecase of a leak or while dual humidity and temperature sensor 38 is beingserviced or replaced.

[0033]FIGS. 4 and 5 show front and rear perspective views of evaporator30, respectively. Refrigerant lines 26 and refrigerant return lines 27are surrounded with thermal insulation to prevent condensation and lossof efficiency. Desiccant slot cover 28 (FIG. 4) encloses desiccant slot37 and supports “D” connectors 98 which connect dual humidity andtemperature sensor 38 to respective refrigeration units 20. Similarly,“D” connectors 99 are provided to connect evaporation block temperaturesensor 39 to respective refrigeration units 20.

[0034]FIG. 6 shows a schematic representation of airflow managementwithin processor cage 100. Air is drawn into processor cage 100 at airinlet 102. Air flows past memory books 105 and redundant power supplies(not shown) along path 103. Air is then forced though one of a pluralityof blowers 110 (only one shown). Blower 110 includes louvers 11 whichoperate as check valves to prevent back flow through blower 110 whenfewer than all blowers are operating. Air flows generally along path 107by memory books 106 and along path 108 by heater 40. A majority of thisair exits at exit 119, however some air flows through orifice 112 andthen past evaporator 30 and then recirculates through one or more ofblowers 110. In this manner, evaporator housing 32 is warmed by airpreviously warmed by absorbing heat from the power supply (not shown),memory books 105, 106, blowers 110, and heater 40. This warmed airpasses heat energy to evaporator housing 32, thereby increasing itssurface temperature to above dew point, ensuring that no condensationwill form thereon.

[0035]FIG. 7 shows a perspective view of cage controller 100. A sourceof pressurized air supplies air through air hose 113 to desiccantcanister 45 (FIG. 1) which resides in canister housing 115. Aview-window 116 is provided so that an operator can see when thedesiccant changes color, indicating saturation. Canister housing 115outputs dry air to capillary tube 104 which is sufficiently long torestrict the air flow as previously described. Capillary tube 104 feedsinto heat spreader plate 41 as shown. Cavity 101 receives modularredundant power supplies which provide power to logic module 50 (FIG.1).

[0036] Referring to FIG. 8, each refrigeration unit controller 24includes a processor 120, an analog-to-digital converter 122, randomaccess memory 124, non-volatile memory 126, communication port 128, andoutput 132. Controller 24 also includes other necessary ancillarycomponents such as power supply, clock, etc., which are not shown forsake of clarity. Non-volatile memory 126 may be any type of machinereadable media such as ROM, PROM, EPROM, EEPROM, Flash, magnetic media,optical media, or other known type of non-volatile memory.Communications port 128 is preferably a standard serial port, as aregenerally known in the industry, such as the RS422 serial port. Each ofthese components are in communication with processor 120 via one or moredata busses 130 (only one shown).

[0037] During operation of controller 24, software stored innon-volatile memory 126 causes processor 120 to perform variousoperations on input and to generate outputs accordingly. As shown inFIG. 1, controllers 24 are in communication with primary and secondarycage controllers 60 and 65. Additionally, cage controllers 60 and 65,which are also intelligent devices, are in communication with othercontrollers and sensors via Ethernet (see line 68 in FIG. 1) asgenerally known and understood in the art. Cage controllers 60 and 65are responsible for monitoring and regulating the power supply, coolingfans, internal processor cage temperature, and other environmentalaspects of the processor cage to ensure proper functioning of the systemand the various internal components.

[0038] The use of an intelligent refrigeration unit controller allowsthe refrigeration unit to cooperate with the cage controller and largersystem to maximize the performance of the logic module withoutsacrificing reliability. In this regard the controller can, in responseto instructions from cage controller 60, precisely control thetemperature of the evaporation block by controlling the speed ofcompressor 70 and by actuating hot gas bypass valve 80 (FIG. 2) inresponse to temperature readings from evaporator block temperature probe39. The temperature to which the evaporator block is controlled may varyaccording to the current condition or operation of the logic moduleoperating mode or power consumption, as well as the expected conditionor operation of the logic module. In addition, the refrigeration unitcontroller is capable of reacting to component failures and notifyingthe cage controller of such failures. This provides improved reliabilityby allowing the cage controller to then switch to a second refrigerationunit and alert system administrators of the failure for repair orreplacement of the defective component and/or refrigeration unit.Furthermore, by integrating the refrigeration unit with the cagecontroller, smooth transitions from one refrigeration unit to anothercan be easily achieved. TABLE 1 SYSTEM RU1 RU2 N mode On Off DeactivatedN + 1 mode On Enabled Switchover mode Switchover-from Switchover-to

[0039] The system and the individual refrigeration units are capable ofentering several different modes of operation to accomplish this task assummarized by Table 1. These modes will now be described with referenceto an exemplary system capable of being connected to only tworefrigeration units at any one time, though it should be understood thatthe system may be adapted to accommodate any number of refrigerationunits. In the “N-mode,” one refrigeration unit is on while the otherrefrigeration unit is either off or deactivated. When in the off-mode,the refrigeration unit is not operating to cool the logic module, but itis actively communicating with cage controller 60. When in thedeactivated-mode, the refrigeration unit is off-line or disconnected. Inthe N+1 mode, the first refrigeration unit is on, while the second is“enabled”. In the enabled mode, a refrigeration unit is not functioningto cool the logic module, and it is in communication with cagecontroller 60. What distinguishes “enabled” from “off” is that whenenabled, the refrigeration unit is ready to step in and turn on, byitself and without instruction from the cage controller, if necessary.The conditions under which an enabled refrigeration unit may turn on byitself include sensing evaporator over-temperature and the cagecontroller didn't turn it on. A third mode of the system is the“switchover” mode. In a switchover mode, one refrigeration unit isdesignated the “switchover-from” unit, indicating that it is in ashut-down sequence, and one refrigeration unit is in a “switchover-to”mode, indicating that it is in a start-up sequence. The shutdown andstartup sequences vary depending on whether the switchover-from unit isexperiencing a fatal error or whether the switchover-from unit isrunning normally or experiencing only a minor error. A minor error isone that can be compensated, an example of which will be described belowwith reference to FIG. 11.

[0040] Each controller 24 makes status data available to the cagecontroller 60, and the cage controller 60 periodically transmits statusdata and instructions to the refrigeration unit controllers 24. Thisperiodic data transmission is called “stuffage” and allows therefrigeration units to react to failures of the cage controllers, whilethe cage controllers' monitoring of refrigeration unit data allows themto react to failures of the refrigeration units in a logical manner. Inthe exemplary embodiment, refrigeration unit controller 24 sends poststhe value of all the sensors of the refrigeration unit, particularly theevaporator and heater block temperatures, flags indicating the mode therefrigeration unit is currently in, and fault data, such as evaporatorcavity over-humidity, heater block over-temperature, etc. Thisinformation is available to the cage controller to read. Every 7 secondsor so, the cage controller 60 stuffs the refrigeration unit with dataincluding the current power of the logic module, ambient temperature,control commands such as turn on/off, prepare for power on, prepare forself test, enter enable mode, enter/exit switch-over-to mode, enter/exitswitch-over-from mode, fault flags control commands to set or clearfault flags in the refrigeration unit controller.

[0041] During operation, cage controller 60 determines the set point forthe refrigeration units. The set point is the desired temperature ofevaporator block, and is dependent on the requirements of the logicmodule and the mode of operation and power draw of the logic module. Forexample, during start up operation, the set point is initially set to ahigh temperature of, e.g., 0° C. which may be designated a standbytemperature. During self-test or periods of low current draw, the setpoint is set to an intermediate temperature to avoid condensation, e.g.,−10° C. During normal operation, the set point is set to a lowtemperature, e.g., −20° C. It may be necessary to raise the set pointduring periods of low power dissipation in the logic module to ensurethe surface temperature of the portion of the circuit board outside thesealed environment does not rise above the dew point.

[0042]FIG. 9 provides a flow chart diagram describing an exemplarystart-up sequence. This represents code in refrigeration unit controller24 and is particularly directed towards starting up the compressorwithout drawing excessive current from the power supply. The procedurebegins at start block 152 and proceeds immediately to block 154 whichinstructs the hot gas bypass valve 80 (FIG. 2) to open to unload thecompressor. Compressor 70 (FIG. 2) is driven to a relatively slow speedof about 2700 r.p.m. The hot gas bypass valve 80 is then pulsed byopening, closing, then opening again, to ensure that it is functioningproperly. By employing pulse width modulation, the hot gas bypass valveis ramped to 100% closed at block 162. At about 81 seconds fromstart-up, the compressor speed is advanced to 50 Hz, which is equivalentto 3000 rpm at block 164. In block 166 PID (proportional, integral,derivative) control of the evaporator block temperature then begins atabout 86 seconds after startup and the heater is powered to 60% on(e.g., using pulse width modulation with about 2 second cycle time).Block 168 begins thermal regulation and at block 170, the logic threadterminates.

[0043] The cage controller prepares for changes in logic state and setsthe set point prior to the change, but not for so long as to createcondensation. For example, in order for the logic module to function atoptimum cycle time, it should already be chilled to its plannedtemperature condition. Therefore, the logic code must first request alow temperature state, then wait for the cage controller andrefrigeration unit to provide that state, e.g., by changing the setpoint. Once the set point is achieved, the refrigeration unit sets astatus bit for the cage controller to read, thus indicating that it hasachieved status. The status bit may be indicative of an evaporator blocktemperature being within a selected range, e.g., 2° C. or 5° C., of theset point. Once the cage controller notices that the status bit is set,it gives permission to the logic code to operate at the faster cycletime. If the cage controller or refrigeration unit notices that thelogic module failed to increase its cycle time within a selected timeframe, e.g., anywhere from 20 seconds to 2 minutes, it will reset theset point to the high or medium temperature, to avoid condensation.

[0044]FIG. 10 represents code in cage controller 60 that is executedprior to “clocks-on” mode, i.e., prior to running logic module at itsoptimum speed. This procedure begins with block 172 and proceedsimmediately to block 174 wherein one of refrigeration units 20 is turnedon. The unit to be turned on may be selected at random, or otherwise. Atblock 176, the set point temperature T is initially set to high, whichmay correspond to 0° C. At block 178, cage controller 60 then waits forthe status bit in refrigeration unit 20 as described above. If all iswell, cage controller 60 sets the set point to a medium value, e.g.,−10° C. as block 180 provides. At block 182, cage controller 60 thenwaits once again for the status bit in refrigeration unit 20. At block184, with the logic module cooled to its intermediate value, cagecontroller 60 signals higher-level code to commence its self test. Thisstep may be skipped at the user's option. If all is well, cagecontroller continues to block 186 where the set point is finally set tothe low temperature, e.g., −20° C. At block 188, cage controller 60again waits for the status bit. Then, at block 190, cage controllergives the higher-level code the go-ahead for clocks-on, and theprocedure terminates at block 192. If clocks-on fails to commence withina selected time-frame, e.g., 2 minutes, then the set point is returnedto the intermediate value to prevent over-cooling of the circuit board.

[0045] Each refrigeration unit controller 24 monitors its individualhumidity sensor or its independent output of dual humidity andtemperature sensor 38 to ensure that it remains within normal limits. Ifthe sensor output is outside the normal limits of the sensor probe,controller 24 sets an error flag indicating that the sensor is “insane”.Higher level system code monitors the error flags from refrigerator unit20. Upon detecting the error, the higher level system code will messagea system operator that the humidity sensor needs to be replaced, asdescribed below.

[0046] Sometimes, a humidity sensor will slowly drift from the correctreading, rather than returning an insane value. To detect a faultysensor that is merely inaccurate, rather than insane, cage controller 60compares the actual sensor values together when one of refrigerationunits 20 indicates an over humidity condition. If the difference betweenthe readings is beyond a miscompare limit, e.g., 5% relative humidity,the higher-reading humidity sensor is flagged as defective and theoperator is requested to replace the dual humidity and temperaturesensor 38. If the difference between the readings is within themiscompare limit, then a dry air breach warning is surfaced and a repairis requested of this condition. In an alternate embodiment, dualhumidity and temperature sensor 38 is replaced with a pressure sensor.This would protect against failures of the seal around evaporator cavity35, but would not insure against moisture diffusion through the variouselastomeric membranes.

[0047] It should be noted that the output of the thermistors in dualhumidity and temperature sensor 38 are used to correct the sensedrelative humidity value for evaporator cavity 35 in accordance with themanufacturer's specifications. It is this temperature-corrected relativehumidity value that is used for fault detection and isolation. Thecorrection is required since the local air temperature around dualhumidity and temperature sensor 38 varies with ambient as well as withthe temperature set point in evaporator 30.

[0048] Concurrent maintenance, i.e., maintenance during continuedoperation of the logic module, of the dual humidity and temperaturesensor 38 (humidity sensor) is accomplished by deactivating the humiditysensor. In this case, “deactivating” the humidity sensor means causingheat cartridge 44 to thermostatically control the board temperature to ahigher than normal value to prevent moisture from forming thereon whenevaporator cavity 35 is opened for the short time needed to replacehumidity sensor 38. Refrigeration unit controller 24 sets a timer whenthe humidity sensor is deactivated and gives a warning signal if therepair procedure takes too long. For example, if the evaporator cavityis opened for more than 10 minutes, an audible alarm is produced, or awarning message is sent to cage controller 60. If the amount of dry airsupplied from desiccant canister 45 is increased while evaporator cavity35 is opened, then this time may be increased. For example, capillarytube 47 may be replaced with a pair of differently-sized orifices, witha valve and actuating mechanism, to allow increased air flow while thehumidity sensor is being serviced.

[0049] While dual humidity and temperature sensor 38 is being replaced,the internal desiccant bag is also replaced so that any moisture thatenters evaporator cavity 35 during this procedure is removed. Once dualhumidity and temperature sensor 38 is replaced, it is “activated” andthe heaters are returned to normal power. Refrigeration unit 20 thenresumes normal monitoring of the relative humidity levels in evaporatorcavity 35.

[0050] Temperature sensors 46 are monitored for sanity in a mannersimilar to the way the humidity sensors are monitored. For example, ifthe heaters are more than 10° C. apart in their temperaturemeasurements, a miscompare fault flag is set, and one or both sensorsare replaced.

[0051] Outputs of temperature sensors 46 are also compared with over andunder temperature limits. These limits are a function of other systemstates or environmental conditions. A temperature outside the rangedefined by the over and under limits, could indicate a faulty heater,connection, or drive circuit. A defect in the running refrigeration unittriggers a switchover to the other (good) refrigeration unit. Thedefective heat cartridge and thermistor is concurrently replaced byfirst deactivating the heater, causing the good running refrigerationunit to thermostatically control the heater to a lower, touchabletemperature. As with the humidity sensor, a timer is set as a result ofthe heater deactivating command. Sufficient time is allotted forreplacing heat cartridge 44 and thermistor 46, but not so much time asto permit moisture to form on critical surfaces. For example, the timermay be set for 10 minutes, after which an alarm sounds.

[0052] Each refrigeration unit 20 controls its own heat cartridge 44,and the heat value is controlled from no heat to full heat, up to 300 W,if heat cartridge 44 is a 300 W heater. Heat cartridge 44 is controlledby pulse width modulation at a frequency fast enough compared to thethermal mass of heat cartridge 44 that prevents temperature cycling andrelated heater failure, e.g., 2 Hz has been found to be a sufficientlyfast frequency. Unique heat values are used for particular power andenvironmental states. In a method described by FIG. 11, the heater poweris reduced for high ambient air temperature conditions. Ambienttemperature is measured within the refrigeration unit enclosure at theinlet to condenser 71 (FIG. 2) using a thermistor. This thermistor istested for sanity in the same manner as temperature sensor 46 describedabove. Starting with block 194 refrigeration unit controller 24 proceedsimmediately to block 196 and determines whether the thermistor at theinlet of condenser 71 is insane. If not, controller 24 proceeds to block198 wherein the condenser inlet temperature is taken as the ambienttemperature, and controller 24 proceeds to block 202. If the thermistoris insane, then the ambient temperature is set to the ambienttemperature from a thermistor mounted on the system frame, which isprovided in stuffage from cage controller 60. A compensation value isadded to this value since the frame temperature will typically beseveral degrees cooler than the air entering condenser 71, due to theenvironment in refrigeration unit 20. Controller 24 then proceeds toblock 202.

[0053] At block 202, controller 24 tests whether the ambient temperatureis less than 30° C. If so, heat cartridge 44 is set to 60% of maximumand blower 72 (FIG. 2) is set to 2,000 rpm. Then, the procedureterminates at block 208. If the ambient temperature is greater than 30°C., the heat cartridge is set to only 30% of maximum and the blower isset to 2,800 rpm, to compensate for warmer ambient air.

[0054] Switchover from one refrigeration unit 20 to anotherrefrigeration unit 20 can occur on a scheduled basis, for example, every160 hours. In addition, switchover can be for recovery purposes, i.e.,when one refrigeration unit is faulty. In particular, a recoveryswitchover may be in response to such failures as double communicationsfault to the on refrigeration unit, temperature sensor 46 insane, overtemperature, or under temperature, or dual humidity and temperaturesensor insane or over humidity. A double communication fault occurs whenboth cage controller 60 and secondary cage controller 65 losecommunication with the on refrigeration unit.

[0055] In a switch-over mode, cage controller 60 posts switching-fromand switching-to bits in stuffage to respective refrigeration units 20,then posts a “switchover” function bit. The switching-from refrigerationunit compressor is set to maximum speed and its hot gas bypass valve isclosed (0%) to prepare for turning on the switching-to refrigerationunit. After a short pause, the switching-to refrigeration unit is turnedon, causing significant heat to be passed to evaporator block 31 duringstartup. The switching-to unit set point is then set a small amountlower (e.g., 1° C.) than the set point of the switching-fromrefrigeration unit, causing the switching-to unit to be taxed to agreater extent as the switching-from unit dumps heat through its hot gasbypass valve in an effort to bring the temperature up to its set point.Switching-to and switching-from refrigeration units are designed to becapable of maintaining this “switch-over” mode indefinitely. However,the switching-from refrigeration unit is kept on for a selected periodof time, e.g., 4 minutes. The switching-from refrigeration unit is thenturned off only after the switching-to unit exhibits no fault or warningconditions. After good status, the cage controller shuts down theswitching-from refrigeration unit, and places it in enabled mode, thenclears the switchover mode and function bit.

[0056] In the case of a recovery switchover, there is no pause inbringing the switching-to refrigeration unit to the set pointtemperature, and once achieved, the switching from refrigeration unit isimmediately shut down.

[0057] When a system is powered off after the logic module has beenchilled to below zero, there is a danger that condensation will formwhen the evaporator is removed to service the logic module. To preventsuch condensation from forming, cage controller 60 sends the runningrefrigeration unit 20 a “prepare-for-power-off” command, causing it toopen its hot gas bypass valve 80, causing high enthalpy refrigerant topass directly into the evaporator and thus rapidly heating theevaporator, as well as the logic module it is attached to. To furtheraccelerate the heating of the chilled logic unit and circuit board, heatcartridge 44 is left on. These steps enable logic module 50 to beserviced in an acceptable timeframe without danger of condensation beingformed on the vulnerable areas.

[0058] While the invention has been described with reference to specificembodiments thereof, it is intended that all matter contained in theabove description and shown in the accompanying drawings be interpretedas illustrative and not limiting in nature. Various modifications of thedisclosed embodiments, as well as other embodiments of the invention,will be apparent to those skilled in the art upon reference to thisdescription, or may be made without departing from the spirit and scopeof the invention as defined in the appended claims.

We claim:
 1. A method for operating a refrigeration unit for cooling a logic module, the method comprising: monitoring an output from a humidity sensor located in an evaporator housing, said evaporator housing also housing an evaporator block that is in thermal communication with said logic module; comparing said output with normal limits and a selected maximum humidity; setting an insanity flag readable by higher-level code indicating that said humidity sensor is insane when said output is outside said normal limits; setting an over-humidity flag readable by higher-level code indicating that humidity in said evaporator housing is above the selected maximum humidity when said output corresponds to a value greater than said selected maximum humidity.
 2. The method of claim 1 further comprising: replacing said humidity sensor without shutting down said refrigeration unit upon detecting said insanity flag.
 3. The method of claim 1 further comprising: comparing at least two humidity sensor readings upon detecting said over-humidity flag to obtain a difference; comparing said difference with a selected miscompare limit; replacing said humidity sensor without shutting down said refrigeration unit when said difference exceeds said miscompare limit; servicing said evaporator housing for a possible seal breach when said difference does not exceed said miscompare limit.
 4. The method of claim 1 further comprising: monitoring a temperature signal from a temperature sensor located proximate a heat spreader plate located on a circuit board opposite said logic module; comparing said temperature signal with selected thermistor insanity limits; setting a thermistor fault flag if said temperature signal is outside said insanity limits; comparing said temperature signal with selected over and under temperature limits; setting an over/under temperature fault flag if said temperature signal is outside said over and under temperature limits; causing a switchover to a second refrigeration unit upon detecting one of said thermistor fault flag and said over/under temperature fault flag; replacing at least one of said heat cartridge in thermal communication with said heat spreader plate and temperature sensor without shutting down said second refrigeration unit after said causing a switchover.
 5. The method of claim 4 further comprising: before said replacing, deactivating said heat cartridge and controlling a second heat cartridge to a touchable temperature; and producing an alarm if said replacing takes longer than a selected time limit. 